Admins can edit any article on an account. They do not need to be added to sites.
Permission for editors are controlled at the site level. An editor can have different permissions on different sites.
Follow the instructions below to change the site authoring permissions for an editor.
Navigate to site > click on Authors
In the ScreenSteps browser app:
- Navigate to the site you want to modify
- Click on Authors
Select the appropriate permissions
You can read more about the authoring permission settings here.